This invention relates to a curable silsesquioxane resin composition that, when cured, produces a cured silsesquioxane resin having high strength and fracture toughness without loss of modulus. This invention further relates to a method for synthesizing the composition and the cured silsesquioxane resin.
Conventional thermoset networks of high cross link density, such as silsesquioxane resins, typically suffer from the drawback that when measures are taken to improve a mechanical property such as strength, fracture toughness, or modulus, one or more of the other properties suffers a detriment.
Canadian Patent No. 691,206 (1964) discloses the use of silica-filled silicone resin/fluid combinations for damping vibrations. The ability of the disclosed silicone resin/fluid compositions to dampen vibrations is illustrated through the measurement of the ratio of Gxe2x80x2, the elastic shear modulus, to Gxe2x80x3, the loss shear modulus. The magnitude of this ratio is indicated as being inversely proportional to the ability of the material to absorb vibration. The ratio of Gxe2x80x2/Gxe2x80x3 of the subject materials is compared to that of compositions prepared without a resin constituent.
The above-described toughened silicone compositions are generally of the types having a fairly low modulus of elasticity. As used herein to describe silicone resins, the term xe2x80x9crigidxe2x80x9d means that the resin material, in its unfilled condition, exhibits a certain xe2x80x9cstiffnessxe2x80x9d characterized by having a Young""s modulus of at least 0.67 GPa. As used herein, the term xe2x80x9cunfilledxe2x80x9d means that no reinforcing fillers, such as carbon or glass fibers or silica powders have been added to the resin.
Another method for increasing toughness of a silicone resin is by modifying the silicone resin with a rubber compound. U.S. Pat. No. 5,747,608 describes a rubber-modified resin and U.S. Pat. No. 5,830,950 describes a method of making the rubber-modified resin. The rubber modified-resin is prepared by reacting an uncured organosilicone resin and a silicone rubber to form a rubber-modified resin. The resin and rubber can be reacted by addition reaction, condensation reaction, or free radical reaction. The resulting rubber-modified resin has a Young""s modulus of at least 6.9xc3x97108 Pa in its unfilled condition. However, strength and toughness of the rubber-modified resin is generally inferior to tough organic polymers and still insufficient for some applications.
Rigid silsesquioxane resins have been employed in applications that take advantage of their heat- and fire-resistant properties. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels. Thus, there exists a need for rigid silsesquioxane resins having increased flexural strength, flexural strain, fracture toughness KIc, and fracture energy GIc, without significant loss of modulus or degradation of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings.
Therefore, it is an object of this invention to provide a curable composition that can be used to prepare a cured silsesquioxane resin having high strength and fracture toughness without loss of modulus. It is a further object of this invention to provide a method for preparing the cured silsesquioxane resin.
This invention relates to a curable silsesquioxane resin composition used to prepare a cured silsesquioxane resin. The cured silsesquioxane resin has improved strength and toughness over known resins. The improvements in strength and toughness were made without significant loss of stiffness. The cured silsesquioxane resin is synthesized by a method of copolymerization of a combination comprising a silsesquioxane precursor and a silyl-terminated hydrocarbon. Copolymerization is carried out by condensation reaction. When the silyl-terminated hydrocarbon is used instead of, or in addition to, a traditional silane or siloxane crosslinker, the resulting cured silsesquioxane resin has unexpectedly high mechanical properties.